1. Field of the Invention
The present invention relates to materials for the selective absorption of solar energy and production thereof. More particularly, it relates to materials for the selective absorption of solar energy and comprises applying anodic oxidation and electrolytic coloring to the surface of aluminum as a substrate, and the production thereof.
Since a large portion of solar energy is occupied by the visible and near infrared wavelength range, desirable materials for absorbing the energy are such that their absorption in the visible and near infrared wavelength range is large. On the other hand, the thermal energy absorbed tends to be emitted from the surface of the materials in the form of radiation energy, most of which is occupied by the infrared wavelength range. Accordingly, desirable materials for accumulating the absorbed energy by minimizing such radiation are such that their reflectance in the infrared wavelength range is large. That is, for the selective absorption of solar energy, there is a demand for the development of materials having a large absorbency in the visible and near infrared wavelength range and a large reflectance in the infrared wavelength range.
Aluminum has the following advantages as a raw material: (1) excellent heat conductivity, (2) light weight, (3) excellent mechanical processability and (4) excellent resistance to corrosion.
2. Description of the Prior Art
Proposals for the use of aluminum which has an electrolytically colored anodic oxide film on the surface thereof, as a material for the selective absorption of solar energy are disclosed, for example, in Japanese Patent Publication (unexamined) Nos. 113434/1975 and 3333/1976. In these proposals, however, an anodic oxide film of about 10.mu. or more in thickness is formed as described in every example of the specification. When such an anodic oxide film having a large thickness has been electrolytically colored, its absorptivity in the visible and near infrared wavelength range is high. However, at the same time, it shows a large absorption in the infrared range as shown in FIG. 1-(a) (in the figure, large absorption is observed in the vicinity of 3.mu. or 8.mu. or more). Such materials are not desirable for selective absorption of solar energy since, even though they absorb solar energy, they emit most of the thermal energy in the form of radiation energy.